The present invention relates generally to a rotary control valve wherein relative rotation between a valve sleeve and a valve spool received therein effectively controls fluid distribution and fluid pressure from a pump to opposite ends of a hydraulic actuator. More particularly, the present invention is directed to incorporation of a preload mechanism into the rotary control valve for enhancing "on-center" feel in vehicular power steering systems. The preload mechanism functions to mechanically interlock the valve sleeve and valve spool of the power steering control apparatus for inhibiting relative rotation therebetween so as to increase tactile reactions to steering inputs at low levels of steering wheel input torque.
For efficient operation of rotary control valve, it is desirable that the control valve be correctly set-up ("balanced") when in its neutral or "centered" condition. Accordingly, the preload mechanism should be operable to rotationally bias the spool and sleeve relative to each other and to the neutral condition in which condition fluid flow, if any, through the control valve presents balanced characteristics. Most rotary control valves used in power-assisted vehicular steering systems provide open-center characteristics in the neutral condition such that a supply of fluid under pressure to an inlet port of the control valve provides pressure drops which are "balanced" across its outlet ports which are in fluid communication with opposite sides of the hydraulic power cylinder and to a fluid return system.
In most conventional vehicular power steering systems, the power steering control apparatus is equipped with a rotary control valve wherein relative rotation between the valve spool and valve sleeve from the centered position in response to steering input torque operates to control the degree of hydraulic power-assist provided. It is known in the power steering art to use a preload device for facilitating improvements in the tactile reaction to low level steering inputs. In general, preload devices function to return the rotary control valve to its centered position upon removal of input torque as well as for inhibiting relative rotative between the valve spool and valve sleeve in response to input torques below an over-ride or preload threshold. As such, at input torque levels below the preload threshold value, the power steering system functions substantially similar to a manual mechanical steering system.
One type of preload mechanism used in power steering systems is commonly referred to as an over-ride detent device. While such over-ride detent devices are typically capable of achieving the required preload requirements, they are often subject to undesirable Coulomb friction resulting in excessive hysteresis of the over-ride threshold. Other known preload mechanism can be generally classified as spring-loaded devices. For example, U.S. Pat. No. 4,823,839 discloses a device wherein a C-shaped compression spring is fixed to the valve sleeve. A first abutment secured to the valve spool is located within a mouth portion of the compression spring while a second abutment secured to the valve sleeve is also located within the mouth portion. Upon application of a steering torque exceeding the preload of the compression spring, relative rotation between the valve spool and valve sleeve occurs wherein the abutments react in opposite directions against the mouth portion of the compression spring for exerting a tensile force thereon. This tensile force tends to enlarge the mouth portion of the compression spring. Unfortunately, to avoid lateral "buckling" of the C-spring in response to such enlargement or "camming-open" the mouth portion, the compression spring must be guided through such motion which results in excessive friction and unwanted hysteresis.